1. Field of the Invention
The present invention relates in general to a method for producing micro-optical components. More particularly, it relates to a method for the manufacture of hemi-cylindrical, and hemi-spherical micro-lenses from templates.
2. Description of the Related Art
The increasing demands in miniaturization and parallel processing of optoelectronic devices and the maturity of the process technologies in micron-scale fabrication have pushed forward the development of micro-lenses and other micro-optical components. Various types of micro-lenses developed to date include refractive lenses, diffractive lenses and mixed refractive/diffractive lenses. Refractive lenses bend or focus a light beam by rules of geometric optics where diffractive lenses alter the path of light based on Fourier optics. Mixed refractive/diffractive lenses typically include refractive lenses having the surface thereof textured with diffracting patterns to correct for chromic aberrations.
Current techniques for fabricating micro-optic components include laser micromachining, polymer island melting, localized UV radiated and heated photothermal glass, ion-beam etching of Silicon or InP, swelling the surface of glass, chemical vapor deposition of SiH4 and NO, ion-beam sputtering, and binary optics techniques such as the use of 2-step Fresnel phase plates, blazed reflection grooves, and a wide variety of other techniques.
Appropriately shaped structures for micro-lenses have been created by molding the shapes from a substrate called a “stamper.” Stampers for micro-optic arrays have been fabricated with a number of techniques, including fabrication of a master with precision computer-controlled diamond turning, photolithography, multiple or single beam laser lithography, laser mastering lathe, or e-beam lithography. The stamper itself is typically the end product of a one or multiple step serial replication of the original master. The micro-optic shaped geometric structure may be created from the stamper, using methods such as compression, injection, or sequential injection/compression molding. The micro-optic structure may be fabricated by a plastic injection and/or compression molding process using the stamper as part of the mold assembly.
Sub-micron scale lenses offer performance advantages in many fields, including faster optical processing, reduced aberrations, and improved signal-to-noise ratios. These features make them highly desirable for a multitude of applications. For example, individual lenses formed on the tips of optical fibers and on diode lasers in addition to arrays of lenses, have been demonstrated with desirable performance characteristics, particularly for coupling into or out of fibers, detectors and diode lasers. Arrays of refractive lenslets have been used to provide efficient coupling from arrays of energy sources to amplifier and detector arrays or to bundles of optical fibers.
Micro-lenses have also been applied to optical data storage media to improve data density. Aspects of this application are described in U.S. Pat. No. 5,910,940 to Guerra, which is herein incorporated by reference in its entirety. FIG. 1 illustrates such a system including an objective lens 20, optical disc 24, and a plurality of micro-lenses 26. The use of an array of micro-lenses 26 in conjunction with objective lens 20 provides a narrower focus on the surface of disc 20 for detection of higher density stored data.
In this embodiment, micro-lenses may be created in much the same way as the pits and grooves of standard CD or DVD disks. A master disk may be produced with the same steps, for instance exposure of a glass disk coated with photo-resist on a laser mastering machine (also called a Laser Beam Recorder or LBR) and subsequent development of the photo resist. Instead of pits or flat-bottomed continuous grooves, the exposure parameters are adjusted to create grooves with a semicircular profile at their bottoms. Such profiles can be generated by modifications of the exposure parameters similar to those which are taught in, for instance, Principles of Optical Disk Systems (p. 194), for combining header pits with a tracking pregroove. A nickel replica of the master, also called a stamper, perhaps removed by a few replication generations, is used in an injection molding machine to form blanks, typically made of polycarbonate, having the same geometry as the master. (If the master is formed using the type of photo-resist that becomes more permanent with light exposure rather than less permanent, an even number of nickel replications will give a blank having the complementary and, in this case, desired geometry.) The grooved polycarbonate blanks are then filled with a high index dielectric followed by the other layers of a standard disk structure. Since the disk is normally viewed through the polycarbonate layer (which is not shown in FIG. 1) by the drive, the high index dielectric presents the desired convex surface to the drive.
Conventional lithographic and etching techniques employed for the fabrication of spherical or cylindrical lenses, having sub-micron features with smooth curved surfaces and with predefined surface patterns, is challenging mainly because these techniques are best suited to produce sloped or flat surfaces, not curved ones. Typical methods for fabrication, such as those used in the semiconductor industry, require fine tuning of exposure and etching parameters for conventional lithographic processes with positive and negative photoresists. These methods are extremely sensitive to variations in the process parameters, which are difficult to determine a priori. For example, a lithographic process may readily suffer from irradiation power variations and/or instabilities. A surface, or feature produced with these techniques may be approximately spherical on average, but locally will tend to show substantial deviations from this preferred shape.
Thus, although techniques have been developed to produce micro-lenses of desired shapes, they are difficult to reliably perform, and often produce lenses having some undesired characteristics.